This invention relates to the field of solid state electronics and, more specifically, to a process of forming compound semiconductive materials.
A number of compound semiconductive materials are considered promising for use in thin-film photovoltaic devices. CuInSe.sub.2 is a good example. It has a band gap (E.sub.g) of 1.04 electron volts (eV) and can absorb almost the entire useful spectrum of sunlight at a thickness of less than 1 micron (um). As disclosed in Mickelsen et al., U.S. Pat. No. 4,335,266, efficiencies of over 10% have been reported in thin-film CuInSe.sub.2 /CdS devices.
CuInSe.sub.2 is a defect structure compound having electrical properties which depend dramatically on its composition. It is not easily doped and, in bulk form, exists in many phases. The conductivity type of the material is a function of the ratio of its constituent elements. The material is p-type and has a low resistivity if it is rich in Cu or Se, whereas it is n-type and relatively highly resistive if it is rich in In or deficient in Se, as compared with its normal stoichometric ratio of elements.
The resistivity of CuInSe.sub.2 for photovoltaic purposes is so highly dependent upon composition that it is quite difficult to control by the usual deposition methods. This relationship is illustrated in FIG. 2, which is a graphical representation of resistivity as a function of Cu to In ratio. The slope of the resistivity curve changes sharply and, therefore, even minor deviations in the Cu/In ratio produce extreme variations in resistivity.
Mickelson et al., supra, describes a method of obtaining a film of CuInSe.sub.2 in two thermal evaporation phases. In the first phase, relatively low resistivity (Cu-rich) p-type material is evaporation deposited at 400.degree. C. as a first film, followed by evaporation deposition at 450.degree. C. of a film of relatively high resistivity (Cu-deficient) n-type material. The homojunction at the interface of the films is then eliminated by diffusion. Although efficiencies of 10% have been reported by this method, the geometry and control mechanisms required for evaporation deposition of the two films are exceedingly complex, and are not readily adaptable to use on a commercial scale or at commercial production rates necessary for low cost solar cells.
Similarly, CdTe/CdS solar cells have been proposed, but the known methods of producing them are not ideal. Cells of this type are described in Tyan, U.S. Pat. No. 4,207,119, and Nakayama et al., "Screen Printed Thin Film CdS/CdTe Solar Cell", Japanese Journal of Applied Physics, Vol. 19, No. 4, pp. 703-712 (1980). In each case, the semiconductive layers are deposited in compound form. The Nakayama method involves screen printing the CdS layer as a paste containing Cd and S, firing the paste to form a film of CdS, and repeating the process with a screen printing paste containing Cd and Te. However, this procedure can produce holes and voids in the films due to evaporation of organic components of the pastes.
CdTe has also been electrodeposited in alloy form, as described in Kroger et al., "Cathodic Deposition of CdTe from Aqueous Electrolytes", J. Electrochem. Soc., pp. 566 through 572 (April, 1978) and Fulop et al., U.S. Pat. No. 4,260,427. However, it is extremely difficult to electrodeposit a multinary semiconductor as a compound, due to the widely different electrodeposition potentials of its constituents. The relative deposition rates of the constituents, and therefore the composition of the deposited film, can be controlled only by careful control of the electrolyte and current density distribution within the bath. However, such control is difficult to accomplish on a commercial scale.
In another context, Cd layers have been deposited on Te substrates, as described in Yokota et al., "Growth of CdTe on Te Substrates by Solid-State Reaction", Japanese Journal of Applied Physics, Vol. 13, No. 9, pp. 1757-1766 (1979). Upon annealing, interdiffusion produced a region of CdTe at the interface between the substrate and the Cd layer.
Also of interest is a process described in Grindle et al., "Preparation and Properties of CuInS.sub.2 Films Produced by Exposing RF-Sputtered Cu-In Films to an H.sub.2 S Atmosphere", Appl. Phys. Lett. 35(1), pp. 24-26 (July, 1979). A film sputtered from an alloy of Cu and In was heated in the presence of H.sub.2 S gas diluted with argon, causing S to enter the film from the gas phase to produce CuInS.sub.2.
None of the prior art processes described above provides the economy and level of control desired in the manufacture of thin film solar cells, particularly multinary materials such as CuInSe.sub.2. Therefore, it is desirable to provide a highly controllable, large volume process for forming very precise compound semiconductive materials from their constituent elements.